There is accumulating enthusiasm and experimental support for repairing damaged cardiac tissue using cell- based therapies. One of the most promising cell sources, supported by substantial pre-clinical data, is bone marrow derived mesenchymal stem cells (MSCs). We have shown that MSCs ameliorate the damage induced by anterior wall myocardial infarction (Ml) in a pig model of left anterior descending coronary artery occlusion. Of great clinical relevance, MSCs may be administered as an allogeneic graft and have received FDA approval to be studied for acute Ml. Several critical mechanistic issues require study in order to rationally advance the clinical development of MSCs as a therapy for acute and chronic cardiac injury. This program of work will examine a series of hypotheses regarding the mechanism of action of MSCs as an agent of cardiac repair. In aim 1, we will test the hypothesis that MSCs stimulate cardiac repair by several mechanisms, including stimulation of endogenous repair mechanisms. We will utilize a well-established porcine model of Ml and administer GFP labeled MSCs. Cardiac repair will be monitored with cardiac MRI and multi-detector CT and cardiac tissue will be submitted to confocal microscopy to quantify cell engraftment, differentiation, and the proliferation of endogenous cell sources. Aim 2 will address several critical issues regarding MSC therapy. We will compare autologous and allogeneic MSCs both at functional and molecular/cellular levels to address whether allogeneic cells are equivalent to autologous cells as an agent of cardiac repair. Additionally, we will test the hypothesis that MSCs are superior to whole bone marrow mononuclear preparations, and finally we will assess various cell delivery methods. In aim 3, we will perform studies in 2 established heart failure (HF) models - chronic ischemic cardiomyopathy and pacing induced HF. MSC efficacy in chronic HF will be assessed using comprehensive evaluation of myocardial performance including cardiac magnetic resonance imaging (MRI), hemodynamic pressure-volume catheterization, and electrophysiology testing. The results of this aim have the potential to broaden the patient population eligible to receive cellular myoplasty. Together, these experiments will advance the translational and mechanistic understanding of cellular myoplasty using MSCs and evaluate whether these cells represent a novel treatment for a wide range of acute and chronic structural heart disease.